2014 On considère un modèle d'Ising frustré généralisé sur un réseau bidimensionnel. Ce modèle est paramagnétique à température nulle mais ferromagnétique pourvu que 0 T Tc. On étudie également l'effet de la dilution sur ce système, et l'on montre que l'ordre à longue distance est rétabli dans le modèle dilué sous certaines conditions de concentration, température et interactions qui sont discutées en comparaison avec la percolation usuelle. Abstract. 2014 A generalized frustrated Ising model on a two-dimensional lattice is considered. This model is paramagnetic at zero temperature but ferromagnetic provided 0 T Tc. The effect of dilution on this system is also investigated, and long range order is shown to be restored in the dilute model under certain conditions involving concentration, temperature and interactions which are discussed in comparison with usual percolation.

Triblock copolymers polystyrene-polyisoprene-polystyrene were studied in dilute solution in a selective solvent, i.e., a nonsolvent for polystyrene and a good solvent for polyisoprene. At a concentration equal to 1.6 X 10-3 g/cm3 triblock aggregates appear. Elastic and quasi-elastic light scattering experiments, as well as intrinsic viscosity measurements, performed on these solutions show a loose structure rather than a micellar structure for the aggregates. These experimental results are also discussed from a theoretical point of view.

We consider the surface of a nearly incompressible polymer melt, extending the usual ground‐state analysis of self‐consistent field theory to describe finite length polymers in the ground‐state potential. To maintain self‐consistency, further corrections to the potential are calculated within linear response theory. From this, we find an excess of ends near the surface, followed by a compensating depletion on the Rg length scale, which relies crucially on the finite compressibility of the melt. The attraction of ends to the surface can be described as resulting from a surface potential for ends with a strength on the order of kBT. Our results address the long‐standing controversies of the distribution of chain ends, the chain‐length dependence of the surface tension, and the interaction between objects immersed in a melt. © 1995 John Wiley & Sons, Inc.

Recent experiments, analytical theory, and simulations have raised and examined the possibility of entropically driven segregation effects in conformationally asymmetric polymer blends. We consider herein a model of surface segregation in a molten blend of two polymers with different flexibilities as characterized by the pure-component parameter β2=R2g/Vmol, where Rg is the radius of gyration and Vmol is the molecular volume of a polymer chain. Analytic solutions to the self-consistent field equations are presented for small deviations of the conformational asymmetry parameter ε=(βA/βB)2 from unity. Even in the absence of enthalpic interactions with the wall, we find an effective exchange surface potential of entropic origin, which can be understood in terms of an imperfect screening of the wall by the self-consistent potential. We find that the more flexible component segregates to the surface, in qualitative agreement with an earlier density functional calculation, but with a different parameterization of the surface potential. For weak conformational asymmetry, the magnitude of the segregation is found to be proportional to (ε−1), and inversely proportional to the bulk screening length of the total monomer density. Our analysis indicates that unlike single-component melts, where reflecting boundary conditions are appropriate, molten blends near a surface are described by an effective mixed boundary condition on the polymer Green’s function G(z,z′;s,s′) of the form ∂zG∝UG, where U is the strength of the surface potential. In the perturbative limit, ‖ε−1‖≪1, this proves equivalent to effective constant flux boundary conditions.